It has heretofore been difficult to obtain both excellent thermal resistance and excellent thermal shock resistance as a molding resin for a large apparatus such as, for example, a large capacity rotary electric machine coil, a transformer coil, etc. Particularly, poor thermal shock resistance yields a defect that cracks develop on curing or by a thermal cycling. In general, a resin having excellent thermal resistance is hard and is low in thermal shock resistance. Therefore, a molding resin for a large apparatus requires thermal resistance of at least class F (155.degree. C) and alicyclic epoxy resins have mainly been used therefor. Also, the thermal shock resistance has been improved by adding an inorganic filler to reduce the coefficient of linear expansion of the whole molding resin and thereby decrease a difference in coefficient of linear expansion between the resins and a molding apparatus. However, this method is limited since the fluidity of the resin composition reduces with an increase in the amount of the filler added.
It has been considered to soften the resin composition by adding a flexibilizer, but there has been in this method a problem that the heat distortion temperature is reduced and the thermal resistance is deteriorated.
German Offenlegungsschrift No. 2,359,386 discloses a thermosetting resin composition consisting essentially of one equivalent of a polyfunctional epoxy compound, 1.5 to 5.0 equivalents of a polyfunctional isocyanate compound and a catalyst. When the thermosetting resin composition is heated to a temperature of 80.degree. C or more, isocyanurate rings and oxazolidone rings are formed and three-dimensional crosslinkage and curing occur. Thus, excellent thermal resistance (class H = 180.degree. C) and excellent mechanical strengths at high temperatures which have never been obtained by prior art thermosetting resins can be obtained.
However, said cured product is unsatisfactory in thermal shock resistance as a molding resin for a large apparatus. Particularly, when an amount as large as 54 percent by volume of a filler was blended and the resulting composition was subjected to thermal shock test according to C-shaped washer method, cracks developed on thermal cycling at 180 to -30.degree. C. However, the addition of such a large amount of a filler is practically questionable since the composition becomes difficult to flow.
German Offenlegungsschrift No. 2,440,953 discloses a process for improving the thermal shock resistance of a thermosetting resin having excellent thermal resistance which comprises using as said polyfunctional epoxy compound component an epoxy group-terminated oxazolidone prepolymer obtained by reacting a disocyanate compound with a stoichiometrical excess of a diepoxy compound. According to this process, such flexibility as required in coating film of paints and varnishes, etc. can be obtained, but sufficient thermal shock resistance as a molding resin for a large apparatus as aimed at in the present invention cannot be obtained. Also, since it is necessary for a molding resin to be solventless and the viscosity of such a prepolymer is too high to be used in the absence of a solvent, the process is practically questionable.